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Milky Way keeps a light grip on speedy neighbours

The Large Magellanic Cloud is a new arrival in the Milky Way’s neighbourhood, according to the new study

(Image: Robert Gendler and Josch Hambsch)

The Milky Way’s two best-known companion galaxies are recent immigrants rather than the long-time neighbours they were thought to be, a new study suggests.

The Large and Small Magellanic Clouds are a pair of nearby dwarf galaxies once thought to have been in orbit around our galaxy for billions of years.

But that picture was shaken up in January 2007 when a team of astronomers announced new measurements of the pair’s motion across the sky made by the Hubble Space Telescope.

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Those observations suggested that the two galaxies are moving too fast to be long-time satellites of the Milky Way and instead are falling into our galactic neighbourhood for the first time (see Speeding dwarfs upset galactic family picture).

The scenario made some astronomers sceptical, however, because it seems to leave too little time to produce a long stream of hydrogen gas seen to trail the two galaxies as a result of interactions with our galaxy.

Updated model

Now, new calculations using an updated model of the Milky Way’s gravity reinforce the conclusion that the Magellanic Clouds are recent visitors. The study was led by Gurtina Besla of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US.

The researchers used an updated model of the distribution of the Milky Way’s dark matter – the mysterious substance that accounts for most of the mass in the universe.

Older studies had assumed the cloud of dark matter around the Milky Way remained relatively dense at large distances from the galactic centre. The large amount of dark matter made it easier for the Milky Way to gravitationally hold onto objects in its vicinity.

But studies of other galaxies suggest that dark matter clouds are actually not so dense at large distances. Besla’s team made new calculations for the Magellanic Clouds assuming that the same was true for the Milky Way.

Elongated orbits

Combined with Hubble’s recent velocity measurements, the new model provides the most accurate picture yet of the relationship between the two galaxies and the Milky Way. This provides fresh evidence that the galaxies are not tightly bound to our galaxy and instead must be first-time visitors to the neighbourhood.

Even before the new study and the one in January 2007, previous measurements clocked their speed at close to that needed to escape the Milky Way’s gravity, suggesting they were travelling on very elongated orbits rather than the more circular and stable ones previously assumed.

“A lot of people don’t want to believe our [Hubble] data,” Besla told New Scienstist. “My point is there are some results here that have nothing to do with the new data.”

But Martin Weinberg of the University of Massachusetts in Amherst, US, says it is too soon to say for sure how long the Magellanic Clouds have been in the neighbourhood. “The evidence one way or the other, in my mind, is not secure,” he told New Scientist.

If the new study is correct, it is difficult to reconcile with the length of the hydrogen gas stream trailing the galaxies, he says. The Magellanic Clouds are losing this stream as a result of gravitational tugs from the Milky Way and collisions with its gas clouds.

Hubble confirmation

But these effects should only happen close to the Milky Way and, if the galaxies are first-time visitors, they would not have had much time to produce the stream. “It is not clear that the stream would cover such a large angular extent,” Weinberg says.

Besla concedes that the stream presents difficulties, but says it may be produced at larger distances from the Milky Way if violent events like supernovae push gas out of the Magellanic Clouds.

Future Hubble observations should help confirm that the higher velocities are correct, she says.

So where will the galaxies go in the future? The Small Magellanic Cloud’s velocity and trajectory are too uncertain to tell, says Besla. But she says friction will probably slow the Large Magellanic Cloud down so much that it will be captured into orbit around the Milky Way.

“It will probably come back around again, but it will be on a really eccentric orbit, so it’s probably not going to come back for a very, very long time,” she says.